﻿using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Ecommerce.Common
{
        public class IdWorker
        {
            //机器ID
            private static long workerId;
            private static long twepoch = 687888001020L; //唯一时间，这是一个避免重复的随机量，自行设定不要大于当前时间戳
            private static long sequence = 0L;
            private static int workerIdBits = 4; //机器码字节数。4个字节用来保存机器码(定义为Long类型会出现，最大偏移64位，所以左移64位没有意义)
            public static long maxWorkerId = -1L ^ -1L << workerIdBits; //最大机器ID
            private static int sequenceBits = 10; //计数器字节数，10个字节用来保存计数码
            private static int workerIdShift = sequenceBits; //机器码数据左移位数，就是后面计数器占用的位数
            private static int timestampLeftShift = sequenceBits + workerIdBits; //时间戳左移动位数就是机器码和计数器总字节数
            public static long sequenceMask = -1L ^ -1L << sequenceBits; //一微秒内可以产生计数，如果达到该值则等到下一微妙在进行生成
            private long lastTimestamp = -1L;

            /// <summary>
            /// 机器码
            /// </summary>
            /// <param name="workerId"></param>
            public IdWorker(long workerId)
            {
                if (workerId > maxWorkerId || workerId < 0)
                    throw new Exception(string.Format("worker Id can't be greater than {0} or less than 0 ", workerId));
                IdWorker.workerId = workerId;
            }

            public long nextId()
            {
                lock (this)
                {
                    long timestamp = timeGen();
                    if (this.lastTimestamp == timestamp)
                    { //同一微妙中生成ID
                        IdWorker.sequence = (IdWorker.sequence + 1) & IdWorker.sequenceMask; //用&运算计算该微秒内产生的计数是否已经到达上限
                        if (IdWorker.sequence == 0)
                        {
                            //一微妙内产生的ID计数已达上限，等待下一微妙
                            timestamp = tillNextMillis(this.lastTimestamp);
                        }
                    }
                    else
                    { //不同微秒生成ID
                        IdWorker.sequence = 0; //计数清0
                    }
                    if (timestamp < lastTimestamp)
                    { //如果当前时间戳比上一次生成ID时时间戳还小，抛出异常，因为不能保证现在生成的ID之前没有生成过
                        throw new Exception(string.Format("Clock moved backwards.  Refusing to generate id for {0} milliseconds",
                            this.lastTimestamp - timestamp));
                    }
                    this.lastTimestamp = timestamp; //把当前时间戳保存为最后生成ID的时间戳
                    long nextId = (timestamp - twepoch << timestampLeftShift) | IdWorker.workerId << IdWorker.workerIdShift | IdWorker.sequence;
                    return nextId;
                }
            }

            /// <summary>
            /// 获取下一微秒时间戳
            /// </summary>
            /// <param name="lastTimestamp"></param>
            /// <returns></returns>
            private long tillNextMillis(long lastTimestamp)
            {
                long timestamp = timeGen();
                while (timestamp <= lastTimestamp)
                {
                    timestamp = timeGen();
                }
                return timestamp;
            }

            /// <summary>
            /// 生成当前时间戳
            /// </summary>
            /// <returns></returns>
            private long timeGen()
            {
                return (long)(DateTime.UtcNow - new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc)).TotalMilliseconds;
            }
        }
}
